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Now EMC has added copy-on-write snapshot software to the list of options you can purchase for Symmetrix. Called EMC Snap for Symmetrix, the software uses only about 30% of the disk space required to make a copy with TimeFinder, EMC's point-in-time cloning software. It works by making a pointer-based virtual copy of a volume, to which it subsequently writes only those blocks of data that are about to be changed in the physical base volume.
But snapshots aren't for every environment, says EMC's Chuck Hollis, vice president of storage platform marketing. While snapshots cost less in terms of disk space, the snapped image relies on the same underlying physical block data, which can cause a performance hit. For example, to perform a backup off of a snapped volume, the backup application simultaneously accesses the same physical spindles as the primary application. The process of writing changed blocks to a snapped virtual volume also takes processing cycles, as does the process of "quiescing" the application. "There is no free lunch," Hollis says.
To minimize these issues, EMC has implemented Snap for Symmetrix to run largely
Snap for Symmetrix isn't the first pointer-based snapshot product for the high end--that distinction goes to IBM's FlashCopy, with its No Copy option. FlashCopy recently received a facelift that coincided with the introduction of IBM's T-REX mainframe. FlashCopy V2 now supports data set level copies and improved capacity management and utilization. Hitachi, meanwhile, does not offer pointer-based snapshot capabilities for its 9900 series, but the company plans to do so in the future, an HDS spokesman says.
This was first published in July 2003